Conventionally, a multiplicity of bearing portions and sliding portions of an internal combustion engine for an automobile are lubricated with oil (lubricant oil), and oil return holes are formed on an upper deck of a cylinder head to drip the oil downwards. For example, as disclosed in Japanese Patent No. 2,661,319, a multiplicity of oil return holes are formed in alternation with the cylinders adjacent to a side wall of the cylinder head.
In such an inline multi-cylinder internal combustion engine mounted in an engine room of a vehicle, in which the cylinder line direction goes along the front-rear direction of the vehicle, since the oil on the upper deck flows swiftly, for example, during rapid acceleration toward the rear side of the engine, the oil hardly drips downwardly from the oil return holes located between the front side of the engine and the center of the engine. Thus, the oil easily accumulates on the upper deck on the rear side of the engine so that the amount of oil that can be circulated in the engine may be temporarily insufficient. In particular, a V-type multi-cylinder internal combustion engine, in which a pair of banks incline at a predetermined angle, has an upper deck on the cylinder heads, each of which inclines from the horizontal plane and therefore, the oil tends to accumulate near the rear side portion of the head side wall of the engine, which is located outside the bank, and at a relatively low level on the upper deck.
In addition to the oil return holes, head bolt bosses where head bolts are inserted to tighten the cylinder head and cylinder block, valve spring seat bosses that receive valve spring seats, and a secondary air passage that supplies secondary air to an exhaust passage, etc., are provided in a tightly packed manner, and therefore in order to secure the oil return performance without causing a reduction in rigidity or strength, or a size enlargement of the engine, it is very difficult to enlarge the aperture of each oil return hole or change its position.